Project Summary Rates of Obesity and Type II Diabetes (T2D) remain high in the United States. The consequences of these rates can be measured in dollars, including treatment costs and lost productivity, and in lower indicators of quality of life. Despite the general knowledge that decreased calorie intake and increased calorie expenditure can lead to weight loss and better glucose tolerance, Americans have not made a significant dent in the rates of these diseases through sustained dieting and exercise. Clearly, new treatments are needed to help combat these diseases. One new approach that has received a lot of attention is to increase a type of fat called brown fat, also known as thermogenic adipose tissue. In mammals there are two types of adipose tissue: brown fat and white fat. Brown fat oxidizes fatty acids as heat energy. White fat stores lipids for future use. Previously, our lab has shown that the JAK/STAT pathway is required for proper differentiation of brown adipose tissue in- vitro, specifically Tyk2-STAT3 signaling. This proposed study focuses on the STAT3 aspect of brown fat differentiation. There is almost nothing known about the targets STAT3 regulates during differentiation of brown fat, however, in the preliminary data we show that inhibiting STAT3 significantly reduces expression of brown fat-selective genes while moderately affecting expression of genes common to brown- and white- fat. The in-vivo consequences from loss of STAT3 in brown fat are unknown, and no role of STAT3 in the development of human thermogenic fat has been reported. We hypothesize that STAT3 plays a major role in the terminal differentiation of brown fat by directly regulating brown fat-selective genes. The aims of this study are to determine: 1) The gene targets regulated by STAT3 by performing ChIPseq on murine brown preadipocytes in-vitro, 2) the disturbances to whole animal metabolism due to loss of STAT3 in thermogenic fat, and 3) the role of STAT3 in the differentiation of human thermogenic fat, specifically if STAT3 regulates similar genes that we will identify in rodent tissue. The answers to these questions will increase our understanding the differentiation mechanism of thermogenic adipose tissue, which would be the first step to realizing a therapeutic approach.